DE68924621T2 - Method and device for producing para-aramid pulp and pulp produced by this method. - Google Patents

Description

Background of the invention

The present invention relates to a method and an apparatus for producing para-aramid pulp and the pulp produced thereby.

The industrial need for para-aramid pulp, such as the poly(p-phenylene terephthalamide) pulp marketed by E.I. du Pont de Nemours and company under the brand name "Kevlar", has been steadily increasing. Due to its high temperature stability, strength and abrasion resistance, para-aramid pulp has been increasingly used for brake linings and seals to replace asbestos with its known health risks. Para-aramid pulp is also used in newly developed papers, laminates and composites for applications where high strength and temperature stability are required.

Most para-aramid pulp is made by first spinning oriented continuous filaments by the dry jet wet spinning process disclosed in U.S. Patent No. 3,767,756 and then mechanically converting the filaments into pulp. However, spinning para-aramids is a costly and complicated process. To briefly explain the process, the polymer is dissolved in 100 percent sulfuric acid to produce an optically anisotropic dope. The anisotropic dope is spun through an air gap into a coagulation bath under carefully controlled conditions. Typically, the spun filaments are also washed and dried prior to mechanical conversion into pulp. It is also generally necessary to use a special fiber cutting device to cut the continuous To cut filaments into uniform short lengths before being converted into pulp.

While attempts have been made to produce para-aramid pulp without first spinning a fiber (see, for example, US-A-4 511 623 or EP-A-0 246 732), a commercially viable process for producing para-aramid pulp in this way suitable for current end-use purposes has not been developed.

Summary of the invention

The present invention provides a process for producing para-aramid pulp and novel pulp produced by this process. The process comprises forming a liquid, actively polymerizing solution containing polymer chains of para-aramid by contacting with stirring substantially stoichiometric amounts of aromatic diacid halide consisting essentially of a para-oriented aromatic diacid halide and aromatic diamine consisting essentially of para-oriented aromatic diamine in a substantially anhydrous amide solvent. In a preferred embodiment of the invention, at least about 80 mole percent of the aromatic diamine is p-phenylenediamine and at least about 80 mole percent of the aromatic diacid halide is terephthaloyl chloride. The liquid solution is subjected, when the inherent viscosity of the para-aramid is between 1 and 4, to an orienting flow which produces an optically anisotropic liquid comprising domains of polymer chains within which the polymer chains of the para-aramid are oriented substantially in the direction of flow. The anisotropic solution is then incubated for at least a period of time sufficient for the solution to become a gel, incubation being commenced when said optically anisotropic liquid solution has a viscosity between 5 and 50 Pa s (50 and 500 poise). and is sufficient to generally maintain the orientation of the polymer chains in the anisotropic solution.

The resulting gel is cut at selected intervals transverse to the orientation of the polymer chains in the gel. The para-aramid pulp can then be isolated from the gel.

According to a preferred embodiment of the present invention, the orienting flow is produced by extruding the solution through a die to obtain an elongated anisotropic solution mass, the extrusion preferably providing an average shear of less than about 100 sec-1. It is particularly advantageous if the average shear is less than about 50 sec-1. In this embodiment of the invention, the incubation is initially carried out while the elongated solution mass is conveyed away from the die at a speed no less than the speed of the mass exiting the die, the mass preferably being deposited on a generally horizontal surface moving away from the die. It is also preferred to continue incubation after gel formation to increase the inherent viscosity and/or to promote increased fibril growth in the pulp so produced. In the preferred embodiment of the invention using an extrusion die, the continued incubation is advantageously carried out after the gel has been cut transversely to facilitate storage of the incubated material.

The para-aramid pulp is isolated from the cross-cut gel by means of, for example, a clay kneader containing an aqueous alkaline solution. In the kneader, the gel is neutralized and coagulated and simultaneously reduced in size to produce a pulp slurry from which the pulp is easily recovered.

According to another preferred embodiment of the invention, the nozzle used in the process for producing the para-aramid pulp is a flow orientation device having an elongated flow path defined by the inner surfaces and a layer of a non-coagulating fluid on the inner surfaces to reduce the contact of the actively polymerizing polymer solution with the inner surfaces and to prevent the formation of deposits and clogging of the flow path. In the flow orientation device according to the invention, the walls which essentially completely form the elongated flow path are porous. In the process according to the invention, the pulp is produced directly from the polymerization reaction mixture without spinning, and special spinning solvents are no longer required. In the most preferred embodiment of the invention, where the para-aramid is the homopolymer poly(p-phenylene terephthalamide), the only chemicals necessary for the process are p-phenylenediamine, terephthaloyl chloride, and, for example, N-methylpyrrolidone and calcium chloride for the amide solvent system. The process is particularly well suited for continuous pulp production on a commercial scale.

The para-aramid pulp of the invention consists essentially of pulp-like short fibers which are made up of bundles of fibrils with a submicron diameter of para-aramid free of sulfonic acid groups and which have an inherent viscosity between 2.0 and 4.5 and a diameter between 1 µm and 150 µm and a length between 0.2 mm and 35 mm. The pulp has a crystallinity index of less than 50, a crystallite size of less than 4 nm (40 Å) and a surface area of less than 2 m²/g. Preferably, the submicron fibrils consist essentially of poly(p-phenylene terephthalamide). The new para-aramid pulp produced by the process can surprisingly be used in a similar way to pulp produced from spun fiber, although the inherent viscosity is lower. than that of pulp produced commercially from spun fibre.

Short description of the drawing

Figure 1 diagrammatically illustrates a preferred method according to the present invention;

Figure 2 is a partially broken away, partial cross-sectional view of a preferred flow orientation device according to the present invention;

Figure 3 is a cross-sectional view of the device of Figure 2 taken along line 3-3.

Detailed description of the preferred embodiments

The process of the invention produces para-aramid pulp. The term para-aramid as used in the present specification is intended to refer to para-oriented wholly aromatic polycarbonamide polymers and copolymers consisting essentially of repeating units of the formula

in which AR₁ and AR, which may be the same or different, are divalent para-oriented aromatic radicals. By the term para-oriented is meant that the chain-extending bonds of the aromatic radicals are either coaxial or parallel and oppositely directed, for example substituted or unsubstituted aromatic radicals including 1,4-phenylene, 4,4'-biphenylene, 2,6-naphthylene and 1,5-naphthalene. The substituents on the aromatic radicals should be non-reactive and, as will become clear below, must not adversely affect the properties of the polymer for the purposes of carrying out the present invention. Examples of suitable substituents are chloro, lower alkyl and methoxy groups. As will also be shown, the term para-aramid is intended to mean para-aramid copolymers of 2 or more para-oriented Comonomers including minor amounts of comonomers in which the acid and amine functions coexist on the same aromatic species, for example copolymers prepared from reactants such as 4-aminobenzoyl chloride hydrochloride, 6-amino-2-naphthoyl chloride hydrochloride, and the like. In addition, the term para-aramid is also intended to encompass copolymers containing minor amounts of comonomers having aromatic moieties which are not para-oriented, such as m-phenylene and 3,4'-biphenylene.

The process of the present invention for producing para-aramid pulp comprises contacting generally stoichiometric amounts of aromatic diamine consisting essentially of para-oriented aromatic diamine and aromatic diacid halide consisting essentially of para-oriented aromatic diacid halide in an amide solvent system to produce a polymer or copolymer according to formula I above. The term "consisting essentially of" is used herein to mean that minor amounts of aromatic diamines and diacid halides which are not para-oriented and of para-oriented aromatic amino acid halides may be used, provided that the properties of the resulting polymer are not substantially altered for the practice of the invention. The aromatic diamines and aromatic diacid halides and para-oriented aromatic amino acid halides used in the invention must be such that the polymer obtained has the typical properties of a para-aramid and forms an optically anisotropic solution as required by the process of the invention and causes the polymerization solution to gel when the inherent viscosity of the polymer is between 1 and 4.

According to a preferred embodiment of the invention, at least about 80 mole percent of the aromatic diamine of p-phenylenediamine and at least 80 mole percent of the aromatic diacid halide of terephthaloyl halide, e.g., terephthaloyl chloride. The remainder of the aromatic diamine may be other para-oriented diamines including, for example, 4,4'-diaminobiphenyl, 2-methyl-p-phenylenediamine, 2-chloro-p-phenylenediamine, 2,6-naphthalenediamine, 1,5-naphthalenediamine, 4,4'-diaminobenzanilide, and the like. One or more of these para-oriented diamines may be used in amounts up to about 20 mole percent with the p-phenylenediamine. The remainder of the aromatic diamine may include diamines which are not para-oriented, such as m-phenylenediamine, 3,3'-diaminobiphenyl, 3,4'-diaminobiphenyl, 3,3'-oxydiphenylenediamine, 3,4'-oxydiphenylenediamine, 3,3'-sulfonyldiphenylenediamine, 3,4'-sulfonyldiphenylenediamine, 4,4'-oxydiphenylenediamine, 4,4'-sulfonyldiphenylenediamine, and the like, although it is typically necessary to limit the amount of such coreactants to about 5 mole percent.

Similarly, the remaining diacid halide can be formed from para-oriented acid halides such as 4,4'-dibenzoyl chloride, 2-chloroterephthaloyl chloride, 2,5-dichloroterephthaloyl chloride, 2-methylterephthaloyl chloride, 2,6-naphthalenedicarboxylic acid chloride, 1,5-naphthalenedicarboxylic acid chloride, and the like. One or mixtures of these para-oriented acid halides can be used in amounts of up to about 20 mole percent together with terephthaloyl chloride. Other diacid halides that are not para-oriented may be used in amounts that normally do not significantly exceed about 5 mole percent, such as isophthaloyl chloride, 3,3'-dibenzoyl chloride, 3,4'-dibenzoyl chloride, 3,3'-oxydibenzoyl chloride, 3,4'-oxydibenzoyl chloride, 3,3'-sulfonyldibenzoyl chloride, 3,4'-sulfonyldibenzoyl chloride, 4,4'-oxydibenzoyl chloride, 4,4'-sulfonyldibenzoyl chloride, and the like.

To reiterate, in the preferred embodiment of the invention, up to 20 mole percent of para-oriented aromatic amino acid halides may be used.

In the most preferred embodiment of the invention, p-phenylenediamine is reacted with terephthaloyl chloride to produce the homopolymer poly(p-phenylene terephthalamide).

The aromatic diamine and aromatic diacid halide are reacted in an amide solvent system, preferably using low temperature solution polymerization techniques (i.e., below 60°C) similar to those taught by Kwolek et al. in U.S. Patent No. 3,063,966 for the preparation of poly(p-phenylene terephthalamide) and by Blades in U.S. Patent No. 3,869,429. Suitable amide solvents or mixtures of such solvents include N-methylpyrrolidone (NMP), dimethylacetamide, and tetramethylurea containing an alkali metal halide. Particularly preferred are NMP and calcium chloride, with the percentage of calcium chloride in the solvent being 4-9% based on the weight of the NMP.

According to the invention, the low temperature solution polymerization is preferably carried out by first preparing a cooled solution of the diamine in the amide solvent containing the alkali metal halide. To this solution the diacid halide is preferably added in two stages. In the first stage the diacid halide is added to the diamine solution cooled to between 0°C and 20°C, the molar ratio of acid halide to diamine being between 0.3 and 0.5. The resulting low molecular weight "pre-polymer" solution is then cooled to remove heat of reaction. In the second stage the remaining acid halide is added to the pre-polymer solution while stirring and cooling the solution if desired. In a continuous process, for mixing the acid halide into the pre- Polymer solution advantageously uses a mixer such as that disclosed in US-A-3 849 074. The second stage of polymerization is suitably carried out in a continuous mixer, all surfaces of which are scraped, while the reaction mixture is cooled to control the rate of reaction. The reaction mixture is sensitive to moisture, as is known in the art, and it is desirable to limit exposure to moist air or other water sources.

For the process of the invention, it is desirable to achieve a carefully controlled reaction rate at least after the inherent viscosity has reached 1.0. Polymerization catalysts are generally not required for satisfactory polymerization, and they should not be used if they make controlling the reaction rate more difficult. Nevertheless, the reaction rate must be sufficiently high to cause the solution to gel within a reasonable time after undergoing orientational flow so that, as will become more apparent below, orientation is not lost prior to gelation, but should not be so high as to prevent adequate control of the reaction. Typical reaction rates may be such that a period of time on the order of 1 to 10 minutes is required for the carefully mixed liquid solution containing all of the reactants to gel into a "soft" gel. In a continuous process, in which a mixer whose entire surfaces are wiped is used to carry out the polymerization, the reaction of a solution containing a certain concentration of reactants can be controlled by regulating the holding time in the mixer and/or the temperature of the solution.

As will become more apparent below, sufficient amounts of the diamine and diacid are used in the polymerization so that the concentration of polymer in the resulting actively polymerizing solution is such that the solution becomes anisotropic after flow orientation and ultimately forms a gel by continued polymerization. However, in general the solubility limits of the reactants in the solvent system should not be exceeded. Preferably amounts of diamine and diacid are used which result in a polymer concentration of between 6% and 13% by weight.

When the inherent viscosity of the para-aramid polymer is between 1 and 4, preferably between 2 and 3.5, and while the reaction is still ongoing, the solution is subjected to orientational flow, resulting in an anisotropic solution in which domains of polymer chains are oriented in the direction of flow. For this process step, it is advantageous to transfer the actively polymerizing solution from the polymerizing device to a device designed to subject the solution to orientational flow. Since the solution continues to polymerize during the transfer, the transfer should be started sufficiently early so that the inherent viscosity of the solution is in the correct range when it is subjected to orientational flow. Moreover, it is generally desirable to start the transfer early so that the final inherent viscosity of the pulp does not exceed 4.5, otherwise the pulp fibers will become thicker and larger, and the length to diameter ratio of the pulp (LID) will be reduced. For continuous processes according to the invention, it is desirable that the device used for the flow device be closely coupled to the polymerizer and receive the solution directly from the polymerizer in order to minimize the size and number of surfaces that come into contact with the solution and on which To keep the risk of deposits forming as low as possible.

According to the method of the invention, the actively polymerizing solution is subjected to orientational flow if the solution is a liquid. At the end of this stage at the latest, the liquid solution is optically anisotropic, i.e., microscopic domains of the solution are birefringent and a bulk sample of the solution depolarizes in-plane polarized light, since the light transmission properties of the solution change with direction. The orientation of the polymer chains within the domains is responsible for the light transmission properties of the solution. As soon as the actively polymerizing solution is subjected to orientational flow, the polymer chains in the solution are oriented in the direction of flow.

To produce orienting flow, the solution is subjected to flow under generally laminar flow conditions in which the solution is subjected to shear or elongational flow. While orienting flow can be produced in many different ways, extrusion through a die to form an elongated mass of solution is preferred because the use of a die allows the process to be carried out on a continuous basis. -

As will become more apparent below, it is preferred to use a nozzle that creates shear flow conditions that subject the solution to an average shear of less than about 100 sec-1. The term "average shear" as used herein is intended to mean the integrated average shear. A low average shear is advantageous because the velocity of the solution extruded from the nozzle can be low, and further advantage is achieved when the average shear is less than about 50 sec-1.

As will be explained in more detail below, the shape of the solution mass is preferably such that it is generally non-flowing after it is formed. To facilitate the practice of the invention for mass production on a continuous basis, the nozzle produces an elongated solution mass whose width is considerably greater than its thickness. Preferably, the nozzle provides a substantially linear flow path and includes a manifold which produces a generally uniform flow across the width of the nozzle.

In the most preferred embodiment of the process of the invention, the die is a flow orienting device having internal surfaces defining an elongated flow path. A layer of non-coagulating fluid is provided on the internal surfaces to reduce contact of the liquid polymer solution with the internal surfaces. Since the actively polymerizing solution has a tendency to settle and clog an extrusion die, this embodiment of the invention is particularly suitable for continuous pulp production as it can minimize deposits in the flow path and help to enable the process to run for extended periods without interruption.

The non-coagulating fluid may be a liquid or a gas which does not coagulate the solution and does not adversely affect pulp yield and quality. To facilitate the generation and control of the layer of non-coagulating fluid, a liquid non-coagulating fluid is preferably used, and it is particularly convenient to use the same liquid solvent system used in the actively polymerizing solution or a liquid component of the solvent system for the actively polymerizing solution so that no new liquid is introduced into the process which increases the complexity the solvent recovery. For example, when NMP and calcium chloride form the solvent system, NMP and calcium chloride can advantageously be used as the non-coagulating fluid, or, due to the absence of salt, NMP alone can also be used.

In the process of the invention, the layer of non-coagulating fluid is sufficiently thick and continuous to form and maintain a lubricating "boundary" layer between the internal surfaces of the device and the solution, which minimizes the formation of deposits. The cross-section of the flow path of the flow orientation device decreases from its inlet to its outlet. Because of the lubricating effect of the layer of non-coagulating fluid on the internal surfaces defining the flow path, orientation of the solution as it passes through the device is primarily due to the stretch flow. The degree of stretch created in the device is large enough to create the orientation necessary to produce pulp in the anisotropic solution. Extremely high degrees of stretch are not necessary and should normally be avoided as they increase the complexity of the process and apparatus employed.

The preferred apparatus of the invention creates the layer of non-coagulating fluid on the interior surfaces by using porous walls which define substantially the entire elongated flow path for the solution. The non-coagulating fluid is forced to exit through the porous walls by being pumped under pressure to a conduit which is in fluid communication with the exterior surfaces of the porous walls. To prevent clogging of the pores of the porous walls, it is necessary that the pressure of the non-coagulating fluid be above the pressure of the solution moving through the flow path. It is desirable that the The pore size of the pore walls is sufficiently small so that no more non-coagulating fluid enters the actively polymerizing solution than is necessary to effectively reduce the deposits. The porous walls may conveniently be made of sintered metal, such as 316 stainless steel, or porcelain, which are resistant to chemical attack by the solution, and preferably define a flow path having a linearly tapered, generally rectangular cross-section.

When an extrusion die, such as the flow orientation device, is used according to the invention, the resulting stretched polymer solution mass extruded from the die is preferably conveyed away from the die at a speed not less than the speed of the mass exiting the die. This can be advantageously done by depositing the stretched mass on a moving, generally horizontal surface, such as a conveyor belt. Since the solution is still a liquid, the solution mass should be moved away at a speed at least equal to the speed of the mass exiting the die, so that orientation within the mass is maintained. It is also necessary that the material flowing onto the conveyor belt is not interrupted by too high a conveyor belt speed, which can have a detrimental effect on pulp quality. The nozzle should be positioned in relation to the conveyor belt so that only a minimal "free fall" of the solution mass from the nozzle onto the conveyor belt occurs, which could disturb the orientation of the polymer chains.

In the preferred nozzle, which defines a linear flow path through the nozzle, the angle of the nozzle flow path with respect to the conveyor belt is such that the mass is deposited cleanly on the conveyor belt without external parts the nozzle adjacent to the nozzle are wetted by the solution. To deposit the solution cleanly on the conveyor belt, the angle between the conveyor belt surface and the flow path should generally be between 90º and 165º. During flow orientation, the temperature should be maintained between 5ºC and 60ºC so that the polymerization reaction proceeds, preferably at a controlled high rate as previously described.

The method comprises incubating the anisotropic solution oriented during flow orientation to cause polymerization to continue for at least a period of time sufficient for the solution to become a gel. By the term "incubating" is meant maintaining conditions that result in continued polymerization and/or fibril growth and that maintain the orientation of the oriented anisotropic solution. As will become apparent later, the incubation conditions may be varied as incubation proceeds.

Incubation is started when the viscosity of the solution is between 5 and 50 Pa s (50 and 500 poise) and is sufficient to generally maintain the orientation of the polymer chains in the anisotropic solution until the liquid solution becomes a gel. The viscosity of the actively polymerizing solution is therefore in a range such that the orientation of the polymer chains in solution does not decrease significantly before the solution gels. The viscosity at the start of incubation can vary within a range which depends on the concentration of the polymer in the solution and on the inherent viscosity of the polymer in the solution. It is believed that a suitable viscosity range at the start of incubation generally corresponds to the viscosity of a poly(p-phenylene terephthalamide)/NMP-CaCl solution having a polymer concentration of between 6 and 13%, the inherent viscosity of the polymers is in the range of 2 to 4. Preferably, the solution viscosities at the beginning of the incubation fall in the range of 15 to 50 Pa s (150 to 500 poise).

In order to maintain the orientation of the polymer chains in the solution as much as possible, incubation is preferably started when the viscosity is sufficiently high and close to the point at which continuation of the reaction will cause the solution to form a gel. It is therefore desirable that the solution be close to the gel point prior to incubation. This is particularly desirable in the preferred embodiment of the invention in which the oriented solution is extruded from the nozzle and deposited on a surface for incubation. In this embodiment of the invention, it is desirable that the solution not flow significantly after orientation and prior to gelation, which would result in loss of orientation. However, the solution viscosity should not be so high that "breaking" of the solution occurs during flow orientation, which could result in poor pulp quality. The temperature can be suitably controlled during flow orientation to adjust the reaction rate to achieve optimal solution viscosities during flow orientation so that the viscosity will be appropriate for the start of incubation. In the preferred embodiment using the extrusion nozzle, an appropriate length for the nozzle is selected and/or the nozzle temperature is adjusted so that the solution is extruded with an appropriate viscosity for the start of incubation.

Incubation is continued for a sufficient time until gel formation occurs. It is desirable that the temperature is between 25ºC and 60ºC until the solution gels in order to maintain a high reaction rate. Most preferably, the temperature is maintained between 40ºC and 60ºC until the solution has become a solid gel. Above 40°C a high reaction rate is achieved and it is believed that above 40°C better pulp formation in the gel also results. In the preferred embodiment employing an extrusion die and conveyor belt, incubation on the conveyor belt is started as soon as the solution is transported away from the die and the solution is conveyed for a period of time sufficient to allow the solution to gel. In order to reduce residence time on the conveyor belt, the solution on the conveyor belt is preferably heated to achieve the above temperature range and thus increase the reaction rate so that gelation on the conveyor belt typically occurs within a period of minutes. Preferably gelation to a hard gel which can be cut as described below occurs within 2 to 8 minutes after the start of incubation. Before the solution gels and while it is a newly formed "soft" gel, it is sensitive to moisture and it is desirable to limit exposure to moist air, such as by creating a dry inert atmosphere of, for example, nitrogen or argon around the incubated solution.

After gelling, the gel is cut at selected sections transverse to the chain orientation. By the term "transverse" is meant any angle of cut that is not parallel to the orientation of the polymer chains. Cross-cutting of the gel is carried out so that the maximum length of the pulp fibers can be controlled. In addition, it is believed that cross-cutting of the freshly gelled solution leads to more uniform pulp fiber lengths, which can result in the production of a more fibrillated pulp with a high surface area. In the preferred embodiment employing the extrusion die, cross-cutting is conveniently carried out by cutting the hardened gel on the belt into discrete pieces using a guillotine-like cutting device, the Cutting rate is coordinated with conveyor speed to determine cut length. Cutting the gel shortly after gelation makes a continuous process using an extrusion die easier because the conveyor length only needs to be long enough to allow time for the solution to gel. Preferably, the gel is cut at intervals of less than about 1/2" (1.27 cm), and is cut when the gel is sufficiently hardened so that the gel pieces do not stick to each other or to the cutting device and are not substantially destroyed during normal handling. The temperature during the cutting operation should preferably be above about 104°F (40°C) to facilitate cutting.

Preferably, incubation is continued after the cutting operation so that polymerization progresses during the continued incubation time so as to increase the inherent viscosity of the polymer. The length of the continued incubation will depend on the length of incubation prior to the cutting operation. A very short additional incubation (or even no additional incubation) may be used if the inherent viscosity upon cutting is already within the range desired for the pulp to be produced. In the preferred embodiment where an extrusion die is used and where it is advantageous to cut the gel shortly after extrusion, continued incubation is highly desirable and may even be necessary to achieve a suitable inherent viscosity for the pulp to be produced. To minimize the time for continued incubation, the temperature is preferably maintained at temperatures above room temperature, preferably between 40-55°C. The time for continued incubation is variable and depends on the desired product, but should generally be longer than about 20 minutes at 40-55ºC if the solution is cut shortly after gelling. Continued incubation affects the size distribution of the pulp produced by the process, because continued incubation combined with the cutting process increases the average length of the pulp-like short fibers in the pulp and their length approaches the cut length of the gel.

In the preferred embodiment of the invention using the extrusion die, the additional incubation can be carried out as a separate process step by storing the cut gel pieces at an elevated temperature and the material can be solidified, for example, in containers or on a slow moving conveyor to reduce the space required during continued incubation. Typically, the hardened gel pieces are stable and it is not necessary to take any special protective measures other than to prevent contact with water and with moist air during continued incubation.

The pulp is isolated from the cut gel after incubation. Isolation is accomplished by size reduction of the material, such as by reducing the gel in size, and by neutralizing and coagulating. To facilitate size reduction, size reduction is accomplished before or preferably simultaneously with neutralizing and coagulating. Size reduction, coagulation and neutralization are conveniently accomplished by contacting the gel with an alkaline solution in a mill or grinder, but it may also be convenient to use a Reitz refiner at this point. The pulp slurry produced is washed, preferably in stages, to remove the polymerization solvent for later recovery. The solvent may be recovered from both the neutralization solution and the wash water for reuse. The pulp slurry is dewatered, for example by vacuum filtration, and optionally dried, such as in a forced air oven, to give products of varying moisture content and so as to meet the requirements of the end use. If desired, the end use pulp can be supplied in a moist, non-collapsed, "never-dried" form containing at least about 30% water by weight of the dry pulp.

Referring now to the drawings, a typical continuous process according to the present invention suitable for commercial production of para-aramid pulp is illustrated diagrammatically in Figure 1. After the second stage of diacid addition to the prepolymer solution, polymerization is carried out in a self-stripping polymerizer indicated by the reference numeral 10. The still polymerizing solution is then discharged into a nozzle 12 for orientation. When the solution is extruded from the nozzle 12, the reaction has progressed to the point where the inherent viscosity is at the desired level through the reaction in the polymerizer 10 and during the residence time in the nozzle 12.

The nozzle 12 subjects the solution to an orienting flow which orients the growing polymer chains in the solution in the direction of extrusion.

Referring now to Figures 2 and 3, there is illustrated a preferred nozzle (extensional flow orientator) 12 in accordance with the present invention. The flow orientator is used in conjunction with a continuous twin screw polymerizer 10 which is fully surface-stripped and has a downwardly directed discharge port 30. A motor and gear (not shown) drive the rotatable screw shafts 37 in the same direction within the polymerization barrel 40 to mix and convey the polymerization solution through the polymerizer. The polymerizer 10 has cooling channels (one is identified by reference numeral 32) so that the temperature of the polymerizer can be conveniently controlled. The polymerizer shown has upper and lower housing sections 34 and 36, respectively, and can be easily disassembled to facilitate cleaning and maintenance. At the outlet port 30, the screw shafts 37 have self-stripping vanes 38 in the barrel 40 which force the contents of the barrel 40 together with the advancing polymer solution out of the outlet port 30. Polymerizers of this type are commercially available, such as that manufactured by Teledyne Readco, York, Pennsylvania.

The flow orientator 12 is closely coupled to the polymerizer 10 and connected to the lower housing section 36 so that the flow orientator 12 receives the actively polymerizing PPD-T solution directly from the barrel 40 of the polymerizer 10. A flow orientator housing 42 having an upper flange portion 44 as shown in Figure 3 is secured to the lower housing section 36 by means of cap screws 45 or other suitable means. Twin screw polymerizers of the type shown generally have a recessed area 46 around the outlet opening 30 on the underside of the lower housing section 36 and the flange portion 44 of the flow orientator housing 42 can be recessed into the recess 46. The vertical positioning of the housing in the recess can be achieved with spacers 48 of suitable thickness.

The extensional flow orientation device 12 includes a flow path 50 and has an inlet 52 at the outlet port 30 of the polymerizer 10, the flow path decreasing in cross-section toward an outlet port 54. The flow path 50 is formed by porous walls 56 defining a rectangular, linearly decreasing cross-section, the width of the nozzle remaining constant as the thickness decreases. The flow path of the device shown is generally intended to be a 90º angle to the conveyor belt 14 (the conveyor belt direction is shown by arrow 57). In the nozzle shown, the thickness decreases at a ratio of about 3 to 1 from inlet 52 to outlet 54 and the nozzle exit 54 has a width about 5 times the thickness.

The porous walls 56 have a layer of N-methylpyrrolidone emanating from the walls. In the illustrated embodiment, this is accomplished by providing an N-methylpyrrolidone supply jacket 58 surrounding the porous walls 56. The jacket 58 is supplied with N-methylpyrrolidone by means of supply lines 62 which come from a pressurized N-methylpyrrolidone source (not shown) and are connected to the housing 42 at connection points 63.

To facilitate construction of the illustrated flow orientation device 12, the porous walls 56 forming the flow path 50 are formed by two porous metal members. Immediately adjacent the cylinder 40 of the polymerizer 10 is a head end piece 64 made of No. 316 stainless steel porous plate material having a pore size of 1.0 to 2.0 microns. The head end piece 64 is machined so that its upper surface conforms to the rotational movement of the blades 38 of the polymerizer 10 at the outlet end 30. The interior of the head end piece 64 is hollow to provide an approximately uniform thickness of the porous wall adjacent the cylinder 40 of the polymerizer 10 and for the flow path 50. The hollow region is in fluid communication with the N-methylpyrrolidone supply mantle 58.

The second portion forms the major portion of the flow path 50 and is formed by a rectangular tapered tubular member 68 which is of unitary construction made of porous 316 stainless steel with a pore size of 0.2 to 1.0. The tubular member 68 is mounted within the housing 42 between the head end 64 and a bottom end 70 having an outwardly tapered opening that corresponds to the exit 54 of the flow path 50. The bottom end 70 is secured to the housing 42 by screws 71 or other suitable means. Lower seals 72 are provided in seal recesses that serve to confine the N-methylpyrrolidone in the supply jacket 58 formed in the space between the outside of the tubular member 68 and the inside of the housing 42. Upper seals 74 are similarly provided between the head end 64 and the housing 42 and between the tubular member 68 and the head end 64 to similarly confine the NMP flow. Contact between the outer surfaces of the head end 64 and the recessed areas of the lower housing section 36 of the polymerizer 10 helps prevent leakage from the porous metal of the head end. Nylon-tipped set screws 76 are mounted in the housing 42 to adjust and secure the position of the tubular member 68.

The resulting elongated strip of oriented anisotropic liquid solution (not shown) is deposited on conveyor belt 14, referring again to Figure 1. At the time the liquid solution is deposited on the conveyor belt, the viscosity is sufficiently high so that the orientation of the deposited solution is not lost before the solution gels. On conveyor belt 14, the elongated strip of solution is incubated at an elevated temperature for a sufficient time so that the solution gels into a hard gel before it reaches cutter 16. Cutter 16 cuts the hard gel into pieces (not shown) having the desired lengths, and the pieces then fall into the containers of a bucket conveyor 18 for continued incubation.

When the inherent viscosity of the para-aramid in the gel pieces in the bucket conveyor 18 has reached the desired level, the gel is discharged into a clay kneader 20 containing a dilute sodium hydroxide solution. In the clay kneader 20, the gel is crushed and simultaneously neutralized and coagulated. The resulting pulp slurry is then transferred to a Reitz refiner 22 for further reduction. The pulp slurry is kept under agitation in a slurry tank 24 and is continuously drawn onto an isolation conveyor 26 for washing. The pulp wet cake is then dewatered in a pulp consolidation station 28 for wet packaging and/or dried and crushed for dry packaging. The solvent in the alkaline solution and the wash water are recycled for reuse.

The pulp produced by the process of the invention consists essentially of short fibrillated fibers of para-aramid, preferably p-phenylene terephthalamide, and comprises bundles of fibrils having a submicron diameter and an inherent viscosity between 2.0 and 4.5. Since the process does not involve spinning from a sulfuric acid solution, the para-aramid is free of sulfonic acid groups. The diameter of the pulp-like fibers produced by this process ranges from less than 1 µm to about 150 µm (microns). The length of the pulp-like fibers produced by this process ranges from 0.2 mm to 35 mm, but never exceeds the sections of the cross-cut gel. The crystallinity index measured by X-ray diffraction is less than 50 and the crystallite size is less than 4 nm (40 Å). The pulp is also characterized by fibrils having a wavy, kinked structure. The surface area of this product measured by gas adsorption techniques is greater than about 2 m²/g compared to that of an equivalent amount of unpulped spun fiber of less than 0.1 m²/g, indicating a high degree of fibrillation. It is believed that the pulp fibers are fibrillated along their length than pulp produced from spun fiber and may adhere more strongly to the matrix in matrix applications. Unless the pulp is dried below a water content of about 30% by weight of dry pulp ("never dried"), the pulp fiber will have a non-collapsed structure that is not present in pulp produced from spun fiber.

When the product is used in end-use articles, such as friction products or seals, it surprisingly shows equivalent performance compared to pulp produced by conventional techniques, such as cutting and shredding spun fiber, although the inherent viscosity is lower than that of commercial pulp produced from spun fiber.

The following examples illustrate the invention using the following test procedures.

Test procedure Inherent viscosity

The inherent viscosity (IV) is defined by the relationship

IV = ln (ηrel)/c,

where c is the concentration (0.5 g polymer in 100 ml solvent) of the polymer solution and ηrel (relative viscosity) is the ratio between the flow times of the polymer solution and the solvent measured at 30°C in a capillary viscometer. The inherent viscosity values given and specified here were determined using concentrated sulfuric acid (96% H504).

Crystallinity index and apparent crystallite size

The crystallinity index and apparent crystallite size for poly-p-phenylene terephthalamide pulp are derived from X-ray diffraction scans of the pulp materials.

The diffraction pattern of poly-p-phenylene terephthalamide is characterized by equatorial X-ray reflections with peaks at about 20º and 23º (2Θ).

As crystallinity increases, the relative overlap of these peaks decreases as the intensity of the peaks increases. The crystallinity index (CI) of poly-p-phenylene terephthalamide is defined as the ratio of the difference between the intensity values of the peak at about 23º 2Θ and the minimum of the valley between the peaks at about 22º 2Θ to the peak intensity at about 23º (2Θ), expressed as a percentage. The crystallinity index is an empirical value and must not be interpreted as percent crystallinity.

The crystallinity index is calculated from the following relationship

Crystallinity index = (A - C) x 100/A-D

where A = peak at about 23º 2�Theta;

C = minimum of the valley at about 22º 2�Theta;, and

D = baseline at approximately 23º 2�Theta;.

The apparent crystallite size is calculated from the measurements of the half-width of the equatorial diffraction peaks at about 20º and 23º (2Θ). The primary apparent crystallite size refers to the crystallite size measured from the primary or lower 2Θ scattering angle at about 20º (2Θ).

Since the two equatorial peaks overlap, the measurement of the half-width is based on the half-width at half height. For the 20º peak, the position of the half-height peak is calculated and the 20�Theta; value for this intensity on the low angle side is measured. The difference between this 2�Theta; value and the 2�Theta; value at the maximum Peak height is multiplied by 2 to give the half-peak width (or "line width").

This measurement corrects only for instrumental broadening; all other broadening effects are assumed to be a result of crystallite size. If "B" is the measured linewidth of the sample, then the corrected linewidth is β.

β = (B²-B²)

where "b" is the instrumental broadening constant. "b" is determined by measuring the line width of the peak located at 28º 2θ in the diffraction pattern of a silicon crystal powder sample.

The apparent crystallite size is given by

ACS = (Kλ)/(β cos �Theta), where

K One is set (unit)

λ is the X-ray wavelength, (here 0.15418 nm (1.5418 Å)

β is the corrected line width in radians means

Θ is half the Bragg angle (half of the 20 value of the selected peak as obtained from the diffraction pattern).

In the measurements of both the crystallinity index and the apparent crystallite size, the diffraction data are processed using a computer program that smoothes the data, determines the baselines, the position and height of the peaks and the position and height of the valleys.

The X-ray diffraction patterns of the pulp samples are determined using an X-ray diffractometer (Philips Electronic Instruments; ct. no. PW1075/00) in reflection mode. The intensity Data are measured with a pulse frequency meter and recorded by a computer-controlled data collection/reduction system. The diffraction patterns are obtained using the following instrumental settings:

Scanning speed 1º 2�Theta; per minute;

Increment step 0.025º 2�Theta;

Scanning range 6º to 38º 2�Theta; and

Pulse height analyzer "Differential".

surface

Surface areas are determined using the BET nitrogen absorption method using a Strohlein surface area meter, Standard Instrumentation, Inc., Charleston, West Virginia. The washed pulp samples are dried in a tared sample flask, weighed and placed on the apparatus. The nitrogen is absorbed at the temperature of liquid nitrogen. The absorption is measured by the pressure difference between the sample flask and a reference flask (gauge readings) and the specific surface area is calculated from the gauge readings, barometric pressure and sample weight.

Measurements of length and diameter

About 5 milligrams of dried and loosened pulp are fiberized and spread out. The fiber lengths and fiber diameters are measured using a magnifying glass with 12x magnification and a precision millimeter reticle with 0.05 mm lines. The resolution is 0.01 mm.

example 1

This example describes the preparation of poly(p-phenylene terephthalamide) pulp in an NMP-CaCl solvent using a laboratory-scale apparatus, where the polymerization is carried out in a batch process and a Couette cylinder device is used for flow orientation. The polymer concentration is 9 wt.% and the CaCl concentration is 5.9%, based on the total weight of the solution.

Prepare a solution of calcium chloride (65.8 g; 0.593 mol) in anhydrous N-methylpyrrolidone (900 mL) by stirring and heating to 85°C to dissolve the calcium chloride. After cooling the solution to 25°C in a round-bottom flask equipped with an overhead stirrer and a dry nitrogen purge, 45.81 g (0.4236 mol) of p-phenylenediamine is added with mixing and the resulting solution is cooled to 10°C. Anhydrous terephthaloyl chloride (TCL) (43.0 g, 0.2118 mol) is added with stirring, causing the temperature to rise to 42.1°C. The solution is cooled to 10ºC and the remaining TCL (43.00 g; 0.2118 mol) is added with vigorous stirring, giving an adiabatic heat gain of about 12ºC. Vigorous stirring is continued as polymerization progresses.

When the still polymerizing solution is translucent at rest and milky when stirred (the inherent viscosity of the poly(p-phenylene terephthalamide) in the mixture is greater than about 1.5), stirring is stopped and the solution is transferred to a Couette cylinder apparatus. The Couette cylinder apparatus comprises an outer tube (inner diameter 10.16 cm (4 inches)) and a coaxial inner cylinder and forms between the outer tube and the inner cylinder an annular tube having a capacity of about 600 cc and a thickness of about 1.59 cm (5/8 inch). The annular tube is equipped with a nitrogen purge and dry nitrogen is fed into the annular tube. The outer tube is provided with a water jacket to control the temperature of the solution in the annular tube and the temperature is adjusted to about 30°C. The inner cylinder rotates at 205 rpm to subject the solution to a shear stress which can be calculated as an average shear of 60 cm⁻¹, with the shear at the inner surface being 81.5 sec⁻¹ and at the outer surface being 38.5 sec⁻¹. When the viscosity approximately 20 Pa s (200 poise) (calculated from the increase in torque at the rotor of the Couette device), the movement of the inner cylinder is stopped.

The water temperature of the Couette water jacket is raised from 30ºC to 50ºC and the solution is incubated at this temperature for 90 minutes. The gel is removed from the Couette and cut into six rings of approximately equal size at different levels in the Couette (T1-82 from the top to the bottom). Each ring was then cut into 6.35 mm (1/4 inch) pieces, with the cut made transverse to the direction of rotation in the Couette cylinder.

The pulp is isolated from the gel by mixing the gel pieces with 5% sodium bicarbonate solution (sufficient gel to produce 10 g of dry pulp and 500 ml of bicarbonate solution) in a Waring blender (about 1800 rpm) for 12 minutes. The pulp material is then dewatered by vacuum filtration. The pulp is then washed twice with water in the blender, followed each time by dewatering. The pulp produced from each of the six rings consists of fine, highly fibrillated fibers having the properties given in Table 1. Table Pulp Properties Sample Inherent Viscosity Diameter (mm) Length (mm) Surface on m²/g

A standard brake compound is prepared using the following composition and molded into 1/2 inch (1.27 cm) molded brake rods at 180ºC for 40 minutes:

50% 200 mesh dolomite

15.2% barium sulfate (BARMITE XF)

15.2% CARDOLITE 104-40

15.2% CARDOLITE 126

3.8% pulp (from samples T1-82)

Flexural strength is measured at room temperature and at 176.7ºC (350ºF) with the following results:

3903 N/cm (5660 psi) at room temperature

2262 N/cm² (3280 psi) at 176.7ºC (350ºF)

Control brake rods of the same composition containing commercially available spun fiber pulp sold under the trademark "Kevlar" by E. I. du Pont de Nemours & Co. give the following flexural strength values:

4151 N/cm² (6020 psi) at room temperature

1324 N/cm (1920 psi) at 176.7ºC (350ºF)

Example 2

The methods and apparatus described in Example 1 are used to produce pulp having the properties set forth in Table 2, except that instead of cutting the T1-82 rings into approximately 1/4" (6.35 mm) pieces, they were cut into various pieces several inches (2.54 cm) long. Table 2 Properties Sample number Inherent viscosity Diameter (mm) Length (mm)

Example 3

This example describes the preparation of poly(p-phenylene terephthalamide) pulp in an NMP-CaCl solvent using a laboratory-scale device and employing batch polymerization and semi-continuous extrusion. The polymer concentration is 10 wt% and the CaCl concentration is 6.5% calculated on the total weight of the solution.

A solution of calcium chloride (42 g; 0.38 mol) in anhydrous N-methylpyrrolidone (500 mL) is prepared by stirring and heating to 90ºC. After cooling the solution to 25ºC in a round-bottom flask with an overhead stirrer and a dry nitrogen purge, 29.3 g (0.271 mol) of p-phenylenediamine is added with mixing and the resulting solution is cooled to 10ºC. Anhydrous terephthaloyl chloride (TC1) (27.5 g; 0.136 mol) is added with stirring, resulting in a temperature rise to 47ºC. After dissolution of TC1, the solution is cooled to 0ºC and the remaining amount of TC1 (27.5 g; 0.136 mol) is added with vigorous stirring until dissolution. Vigorous stirring is continued during the resulting polymerization.

When the still polymerizing mixture is translucent at rest and milky when stirred (the inherent viscosity of the poly(p-phenylene terephthalamide) in the mixture was greater than about 1.5), the solution is flow oriented by pumping it from the round bottom flask at a flow rate of about 2.75 cc/sec through a nozzle having a linear flow path 4 cm wide, 4 mm thick, and 45 cm long to obtain an elongated mass of optically anisotropic viscous liquid. Shear rates in the nozzle range from 0 sec-1 in the central plane of the flow path to a maximum of about 30 sec-1 at the walls of the nozzle (average shear about 15 sec-1). The temperature of the nozzle is maintained at about 25°C. The exit of the nozzle is located about 0.6 cm above a horizontal conveyor belt surrounded by dry warm nitrogen heated to about 50°C, and the oriented anisotropic liquid solution is deposited on the conveyor belt for incubation. The conveyor belt has a maximum travel distance of about 45 cm. The nozzle is inclined with respect to the conveyor belt so that an angle of 1150 is formed between the nozzle and the conveyor belt moving away from the nozzle. The extrusion speed and the conveyor belt speed were both maintained at about 1.7 cm/sec. The width of the conveyor belt is the same as the width of the nozzle (4 cm) and has raised edges to prevent the solution from flowing in a direction perpendicular to the direction of conveyor belt movement. The thickness of the solution on the conveyor belt is about 3 mm. The viscosity of the extruded solution is estimated to be 20 to 30 Pa s (200-300 poise). The conveyor belt and extrusion are stopped when the end of the conveyor belt is reached.

The solution is kept incubated under a heated nitrogen atmosphere (55°C) for about 90 minutes until it becomes a hard gel and for the reaction in the gel to proceed to achieve the desired inherent viscosity. After incubation, the gel is cross-cut into two pieces, designated "L1" and "L2" in Table 3 below, where L1 designates the portion of the gel that was first extruded. Each piece is then cut into several pieces several inches long for isolation of the pulp.

Pulp is isolated from the fully incubated hardened gel pieces according to the following sequence. The gel pieces are mixed with 5% sodium bicarbonate solution (enough gel to produce 10 g of dry pulp and 500 ml of bicarbonate solution) in a Waring blender at high speed (approximately 1800 rpm) for 12 minutes. The pulp material thus isolated was filtered by vacuum filtration dewatered. The pulp is washed twice with hot water in the mixer, each time followed by dewatering. The pulp thus produced consists of fine, highly fibrillated fibers and has the properties given in Table 3. Table 3 Pulp properties Inherent viscosity Diameter of fibres Length of fibres Surface

The pulp is incorporated into a standard brake mixture, formed into bars and tested according to the procedures of Example I, obtaining the following flexural strength values:

3664 N/cm (5314 psi) at room temperature

1278 N/cm (1854 psi) at 176.7ºC (350ºF)

Example 4

This example describes the preparation of poly(p-phenylene terephthalamide) pulp in an NMP-CaCl solvent using the same batch polymerization and semi-continuous extrusion equipment as in Example 3. The gel pieces L1 and L2 are cut into 6.35 mm (1/4 inch) wide strips at an angle of 90° to the length of the gel after incubation and before pulp isolation. The polymer concentration is 7% by weight and the CaCl concentration is 3.8% based on the total weight of the solution.

Prepare a solution of calcium chloride (24.30 g; 0.22 mol) in anhydrous N-methylpyrrolidone (540 mL) with stirring and heating to 75ºC. After cooling the solution to 25ºC in a round-bottomed flask fitted with an overhead stirrer and purged with dry nitrogen, 20.24 g (0.1872 mol) of p-phenylenediamine was added with mixing and the resulting solution was cooled to 10°C. With stirring, anhydrous terephthaloyl chloride (TCL) (19.00 g; 0.0936 mol) was added, causing a temperature rise to 35.3°C. After dissolution of the TCL, the solution was cooled to 5°C and the second portion of TCL (19.00 g; 0.0936 mol) was added with vigorous stirring until dissolved. Vigorous stirring was continued during the resulting polymerization.

When the still polymerizing solution is translucent at rest and milky upon stirring (the inherent viscosity of the poly(p-phenylene terephthalamide) in the mixture was greater than 1.5), the solution is flow oriented by pumping it from the round bottom flask at a flow rate of about 1.85 cc/sec through a nozzle having a linear flow path 4 cm wide, 4 mm thick, and 45 cm long to form an elongated mass of optically anisotropic viscous liquid. Gravity velocities in the nozzle range from sec -1 in the central plane of the flow path in the nozzle to a maximum of about 30 sec -1 at the walls of the nozzle (average shear 15 sec -1). The temperature of the nozzle is maintained at about 25°C. The exit of the nozzle is located about 0.6 cm above a horizontal conveyor belt enveloped in warm dry nitrogen heated to about above 45°C, and the oriented anisotropic liquid solution is deposited on the conveyor belt for incubation. The conveyor belt has a maximum travel length of about 45 cm. The nozzle is inclined with respect to the conveyor belt so that an angle of 115°C is formed between the nozzle and the conveyor belt moving away from the nozzle. The extrusion rate is estimated to be about 1.25 cm/sec, and the speed of the conveyor belt is maintained at about 1.35 cm/sec. The width of the conveyor belt is the same as the width of the nozzle (4 cm) and has raised edges to prevent the solution from flowing in one direction. which is perpendicular to the direction of movement of the conveyor belt.

The viscosity of the extruded solution is estimated to be about 30 Pa s (300 poise). The thickness of the solution on the conveyor belt is about 2 to 4 mm. The conveyor belt and the extrusion are stopped once the end of the conveyor belt is reached.

The solution is incubated under a heated nitrogen atmosphere (45ºC) for about 120 minutes until it becomes a hard gel and the reaction in the gel proceeds. The gel is cut into two pieces "L1" and "L2", where L1 is the part of the gel that was first extruded. The gel is then cut into strips about 6.35 mm (1/4") wide at an angle of 90º to the length of the gel.

Pulp is isolated from the fully incubated and hardened gel strips according to the following sequence. The gel pieces are mixed in a Waring blender at high speed (1800 rpm) for 12 minutes with 5% sodium bicarbonate solution (enough gel to produce 10 g of dry pulp and 500 ml of bicarbonate solution). The pulp material thus isolated is dewatered by vacuum filtration. The pulp is washed twice with hot water in the blender, each time followed by a dewatering step. The pulp thus produced consists of fine, highly fibrillated fibers and has the properties given in Table 4. Table 4 Pulp properties Inherent viscosity Diameter of fibres Length of fibres Surface area (m²/g)

Example 5

This example describes the preparation of poly(p-phenylene terephthalamide) pulp in an NMP-CaCl solvent using the same equipment as in Example 4 for batch polymerization and semi-continuous extrusion.

The procedures of Example 4 are followed except that the gel is cut transversely prior to continued incubation as described in the following paragraph. The polymer concentration is 7 wt% as in Example 4 and the concentration of CaCl is 3.8% based on the total weight of the solution.

The solution is incubated under a heated nitrogen atmosphere (50ºC) for approximately 8 minutes (from the time the solution is deposited on the conveyor until it is cut) until it becomes a hard gel. The gel is cut into two pieces "L1" and "L2" and then cut into strips approximately 6.35 mm (1/4") wide at an angle of 90ºC to the length of the gel. To allow the reaction to continue in the gel, incubation is continued at 50ºC for approximately 110 minutes.

The pulp thus produced consists of fine, highly fibrillated fibres and the properties are given in Table 5. Table 5 Pulp properties Inherent viscosity Fiber diameter Fiber length

Example 6

This example discloses a process for producing poly(p-phenylene terephthalamide) (PPD-T) pulp using an elongated flow orientation porous wall filtration device that provides a layer of N-methylpyrrolidone on the inside of the flow path walls to minimize deposit formation.

An elongated flow orientator having a linearly tapered rectangular flow path made of porous metal plates is mounted to the outlet of a 5-inch (12.7 cm) twin-screw polymerizer having all surfaces stripped and a surrounding jacket but operating without a cooling liquid. The flow orientator has a vertically downward flow path and has an inlet measuring 0.44 inches x 1.9 inches (1.12 cm x 4.83 cm) to directly receive material discharged from the polymerizer, a length of approximately 2.5 inches (6.35 cm), and an outlet measuring 0.23 inches x 1.9 inches (0.58 cm x 4.83 cm). The porous plates forming the walls are 316 stainless steel porous plates and are approximately 0.318 cm (0.125 inches) thick and have a porosity of 0.2 to 1.0 µm (microns). The plates are held in a housing with suitable conduits provided to deliver N-methylpyrrolidone to the outer surfaces of the plates.

An actively polymerizing 9.2 wt.% poly(p-phenylene terephthalamide) solution in N-methylpyrrolidone (NMP) and calcium chloride is discharged from the polymerizer (the molar ratio of CaCl to the original amount of p-phenylenediamine is 1.38). While the PPD-T solution is still polymerizing, it is extruded from the flow orientation device at a polymer flow rate of 12.3 pH. The inner surfaces of the porous walls are continuously supplied with a layer of NMP extruded from the porous metal plates at a flow rate of approximately 0.26 ml/cm2/min (1.7 ml/sq. in./min.) based on the total area of the porous plates exposed to the PPD-T solution. The inherent viscosity of the poly(p-phenylene terephthalamide) in the solution exiting the flow orientation device is approximately 2.3.

The viscous, but still liquid, solution exiting the flow orientation device is periodically collected on a horizontal plate with the plate being moved beneath the outlet at a speed approximately equal to the speed of the solution exiting the flow path. The solution, extruded into strips approximately 5.1 cm (2 inch) wide, is incubated on the plate at ambient conditions and gels to a soft gel in approximately 40 seconds. The gel is then cut into 0.95 cm (3/8 inch) pieces across the direction of flow. The 15 cut pieces are then placed in an oven at approximately 44°C for further incubation for one hour.

To isolate the pulp, the incubated pieces are placed in water in a Waring blender and stirred at high speed for several minutes. The pulp is collected on a filter five times in alternating cycles and returned to the blender for brief stirring with water. The isolated pulp product consists of highly fibrillated PPD-T pulp with an inherent viscosity of 3.1.

Example7

The same equipment and procedures are used for solution preparation and extrusion as in Example 6, except that the extruded solution is produced at a polymer flow rate of 5.62 kg (12.4 pounds) per hour and the N-methylpyrrolidone flow rate is 0.68 ml/cm²/minute (4.4 ml/sq. in./minute). The polymerization and extrusion are carried out over a period of 5 hours. The flow path of the flow orientator remains essentially free of any debris during the 5 hour run, with occasional minor partial blockage. near the flow path exit, which can easily be removed mechanically and the flow path can be completely reopened.

Example 8

This example describes the preparation of poly(p-phenylene terephthalamide) pulp in an NMP-CaCl solvent using a pilot-scale continuous production device.

A p-phenylenediamine solution in NMP-CaCl at a temperature of 10°C containing 5.5% p-phenylenediamine, 7.4% CaCl, 87.1% NMP and less than 200 ppm water is fed to a mixer and mixed with an amount of molten TCL equal to 35% of the stoichiometric amount. The resulting prepolymer is pumped through a heat exchanger to cool the prepolymer to about 5°C. The prepolymer is then mixed with molten TCL at a rate such that a stoichiometric balance is achieved between the TCL and diamine in the mixture using an apparatus such as disclosed in U.S. Patent 3,849,074. This mixture is continuously passed through a 2-inch continuous jacketed twin-screw polymerizer, which is operated without a cooling liquid. Amounts of reactants are used such that PPD-T is produced at a rate of about 4.5 kg (10 lbs) per hour.

The liquid solution from the polymerizer flows directly into a close coupled flow orientator and then onto a continuous conveyor belt to carry away the extruded material. The flow orientator and polymerizer are of the type described in Figures 2 and 3 and have porous walls defining an elongation flow path, an inlet to the flow path having dimensions 1.91 x 3.18 cm (0.75 x 1.25 inches), an outlet measuring 0.64 x 3.18 cm (0.25 x 1.25) inches, and a flow path length of 11.4 cm (4.5 inches). N-methylpyrrolidone is fed to the flow orientator at a flow rate sufficient to form and maintain an interface between the porous walls and the solution. The conveyor belt is 20.3 cm (8 inches) wide, has a length of about 40 feet, and is generally horizontal. The conveyor belt surface is about 1.27 cm (1/2 inch) below the flow path exit, and the angle of the flow orientator flow path with respect to the conveyor belt surface is 90º. The entire conveyor belt area and the flow orientator exit are enclosed and enveloped in nitrogen heated to 45ºC. A strip of solution approximately 3.18 cm (1.25 inch) wide is extruded from the apparatus at a speed of approximately 3.57 m/min (11.7 ft/min) and the conveyor belt speed is also approximately 3.57 m/min (11.7 ft/min)

The solution strip hardens after traveling 35 feet (10.67 m) on the belt (about 3 minutes). A guillotine cutter, timed to the conveyor speed, is located 3 inches (7.6 cm) from the end of the horizontal surface of the conveyor, and the cutter cuts the gel into approximately 1/4" (0.64 cm) pieces at 90º to the length of the gel. The gel pieces that reach the end of the horizontal portion of the conveyor fall into 5 gallon (18.9 l) containers.

Once the containers are filled, they are placed in an oven at 45ºC for 60 minutes to continue incubation.

The containers are periodically removed from the furnace and emptied into small capacity clay kneaders (about 94.6 litres (about 25 gal)) which are supplied with a dilute sodium hydroxide solution. In the clay kneader, neutralization and coagulation take place simultaneously with a first crushing. The Discharge from the clay kneader is continuously fed to a refiner for further reduction. The discharge from the refiner is then transferred to a slurry tank in which a volume of about 757 l (about 200 gallons) of slurry is held under agitation. From the slurry tank, slurry is continuously deposited on a horizontal filter (length 10.67 m (35 feet) and width 43.2 cm (17 inches)) where the pulp is alternately washed and dewatered in vacuum twelve times. The resulting wet cake is then continuously dried in a steam-heated drum dryer.

The pulp produced consists of fine, highly fibrillated pulp with a range of diameters of less than 0.15 mm, a length of less than or equal to about 6 mm and a surface area of greater than 4.0 m²/g.

Claims (20)

1. A process for producing para-aramid pulp, comprising: forming a liquid, actively polymerizing solution containing para-aramid polymer chains by contacting with stirring substantially stoichiometric amounts of aromatic diacid halide consisting essentially of a para-oriented aromatic diacid halide and aromatic diamine consisting essentially of para-oriented aromatic diamine in a substantially anhydrous amide solvent system; subjecting said liquid solution, when the inherent viscosity of the para-amide is between 1 and 4, to orientational flow which produces an optically anisotropic liquid solution comprising domains of polymer chains within which the polymer chains of the para-aramid are oriented substantially in the direction of flow; incubating said anisotropic liquid solution for at least a period of time sufficient for said anisotropic solution to become a gel, incubation beginning when the optically anisotropic liquid solution has a viscosity of between 5 and 50 Pa x 5 (50 and 500 poise) and sufficient to generally maintain the orientation of said polymer chains in said anisotropic liquid solution until said liquid solution becomes a gel; cutting said gel at selected intervals transverse to the orientation of the polymer chains in said gel; and Isolating the para-aramid pulp from said gel. 2. The method of claim 1, wherein said subjecting said solution to orienting flow is accomplished by extruding said solution through a die to produce an elongated anisotropic solution mass. 3. The method of claim 2, wherein said step of incubating said anisotropic solution until said solution becomes a gel is initially performed while moving said extended anisotropic solution mass away from said nozzle at a velocity not less than that of the mass exiting said nozzle. 4. A method according to claim 2, wherein said incubating is initially carried out by depositing the extended mass of solution on a generally horizontal surface moving away from said nozzle at a velocity not less than the velocity of the mass exiting said nozzle. 5. The method of claim 1, wherein said liquid solution is subjected to orienting flow to form said anisotropic solution when the inherent viscosity of the para-aramid is between 2 and 3.5. 6. The method of claim 1, wherein said incubating of said gel is continued after cutting said gel transversely. 7. The process of claim 1, wherein said liquid actively polymerizing solution is formed by dissolving said aromatic diamine in said solvent system and adding between 30% and 50% of said aromatic diacid halide to form a prepolymer solution, and then adding the remainder of the stoichiometric amount of the said diacid halide is added. 8. The process of claim 1 wherein said diacid halide and said diamine are contacted in amounts sufficient to produce a final para-aramid concentration of between 6 and 13 weight percent in said solvent system. 9. A method according to claim 1, wherein said incubation is carried out at a temperature between 25°C and 60°C. 10. The process of claim 1 wherein at least 80 mole percent of said aromatic diamine is p-phenylenediamine and at least 30 mole percent of said aromatic diacid halide is terephthalic acid halide. 11. The process of claim 1 wherein said aromatic diamine is p-phenylenediamine and said aromatic diacid halide is terephthalic acid halide. 12. The method of claim 1, wherein the step of subjecting said liquid solution to orienting flow is accomplished by extruding said liquid solution through an elongated flow orienting device having interior surfaces defining an elongated flow path and providing a layer of non-coagulating fluid on said interior surfaces of said elongated flow orienting device during said extrusion to reduce contact between said liquid solution and said interior surfaces. 13. The method of claim 12 wherein said non-coagulating fluid comprises said solvent system or an amide used in said solvent system. 14. The method of claim 12 wherein said solvent system comprises N-methylpyrrolidone and calcium chloride and said non-coagulating fluid is N-methylpyrrolidone. 15. The method of claim 12, wherein said elongate flow orientation device has porous walls providing openings in said interior surfaces and said non-coagulating fluid is caused to exit from said porous walls into said flow path to provide said layer of non-coagulating fluid. 16. The method of claim 12, wherein substantially all of the interior surfaces defining said flow path are formed by said porous walls. 17. An elongated flow orientation device for forming an elongated, optically anisotropic, liquid para-aramid solution mass containing domains of polymer chains within which the polymer chains of the para-aramid are substantially oriented in the direction of the length of said mass, said flow orientation device comprising: a housing; an elongated flow path within said housing having an inlet and an outlet on said flow path which decreases in cross-sectional area from said inlet to said outlet; means for conveying a solution of said para-aramid to said inlet of said flow path; porous walls within said housing having inner and outer surfaces, the inner surfaces of the said porous walls essentially define the entire elongated flow path; non-coagulating fluid supply means for providing a non-coagulating fluid to said solution in said housing; and Conduit means in said housing for providing fluid communication between said non-coagulating fluid supply means and said outer surfaces of said porous walls, causing said non-coagulating fluid to exit said porous walls and form a layer of non-coagulating fluid on said inner surfaces to reduce contact of said solution with said inner surfaces while subjecting said solution to elongated flow in said flow path. 18. Para-aramid pulp consisting essentially of pulp-like short fibers comprising bundles of submicron diameter fibrils of para-aramid which are free of sulfonic acid groups and have an inherent viscosity of between 2.0 and 4.5, a diameter of between 1 µm to 150 µm and a length of between 0.2 mm and 35 mm, a crystallinity index of less than 50, a crystallite size of less than 4 nm (40 Å) and a surface area of more than 2 m²/g. 19. The pulp of claim 18 wherein the para-aramid is poly(p-phenylene terephthalamide). 20. Pulp according to claim 18, wherein the fibers are uncollapsed and never dried and contain at least 30% water based on the weight of the dry fiber, and after drying have a crystallinity index of less than 50, a crystallite size of less than 4 nm (40 Å) and a surface area of more than 2 m²/g.